Incrementing an undef value is such a common annoyance that recent releases of perl special-case it. It seems to have been documented somewhere between version 5.004 and 5.8.8. Quoting directly from perlop, which is currently for version 5.10.0,

The auto-increment operator has a little extra builtin magic to it....

undef is always treated as numeric, and in particular is changed to 0 before incrementing (so that a post-increment of an undef value will return 0 rather than undef).

It's funny, I've gotten so accustomed to this behavior I'd stopped thinking of it as a special case, though I think you could make an argument that it the behavior itself isn't actually, though maybe the lack of warning is.

Let's be clear about what we mean by "recent" though. The behavior is both present and documented in Perl 5.00503 which was released in 1999.